Over 9 million Americans have symptomatic osteoarthritis (OA) of the knee, a chronic disease associated with frequent joint pain, functional limitations, and quadriceps weakness that intrude upon everyday life. At least half of those with OA of the knee are diagnosed with hypertension or high blood pressure (HBP), one of the most prevalent risk factors for cardiovascular disease. Many other individuals with OA of the knee unknowingly have HBP and remain untreated. Our own work and that of others suggest that persons with OA of the knee experience reductions in BP when they participate in a regular regimen of physical activity. Even small decreases in systolic and diastolic BP found with physical activity are clinically significant, e.g., a 2 mm Hg decrease reduces the risk of stroke by 14% - 17%, and the risk of coronary heart disease is reduced by 6% - 9%. Yet, only 15% of persons with OA and 47% with HBP engage in regular physical activity. The purpose of this study is to investigate how the individually delivered, home-based, 6-month modified Staying Active with Arthritis (STAR) intervention, guided by self-efficacy theory and modified to address comorbid HBP, affects lower extremity exercise (flexibility, strengthening, and balance), fitness walking, functional status, BP, quadriceps strength, pain, and health-related quality of life (HRQoL) in a convenience sample of 224 adults age 50 years or older with OA of the knee and HBP. Using a randomized controlled, 2-group design, we (1) hypothesize that at the end of the 6-month intervention period and 6 months after the intervention period ends those who receive the modified STAR intervention will be more likely to perform lower extremity exercise, participate in fitness walking, show improvements in objective functional status, and demonstrate reductions in BP than those who receive attention-control. Secondarily, we will (2) evaluate the impact of the modified STAR intervention, compared to attention- control, on subjective functional status, quadriceps strength, pain, and HRQoL at both time points;(3) explore the impact of the modified STAR intervention, compared to attention-control, on self-efficacy and outcome expectancy at both time points;(4) explore the relationship between self-efficacy and outcome expectancy;and (5) explore the extent to which self-efficacy and outcome expectancy mediate the relationship between the modified STAR intervention and performance of lower extremity exercise and participation in fitness walking. Data will be analyzed using repeated measures modeling. PUBLIC HEALTH RELEVANCE: The proposed study is relevant to public health because it examines the modified Staying Active with Arthritis (STAR) program to improve leg exercise, fitness walking, and clinical outcomes (function, blood pressure, leg strength, pain, and health-related quality of life) in older Americans with osteoarthritis of the knee and high blood pressure. The modified STAR program addresses the barriers to physical activity from osteoarthritis of the knee as well as high blood pressure related physical activity concerns. The modified STAR program has the potential to reduce the risk of heart disease in the 5 million older adults who have both osteoarthritis of the knee and high blood pressure and who do not engage in the recommended amount of physical activity.